When a manned mission to Mars returns to Earth, it soon becomes apparent
that the three beings who disembark are not the ship's astronauts. The
Doctor realises that the crew have made contact with an alien force on the
Red Planet, but his investigations are interrupted when the aliens
masquerading as the astronauts are kidnapped by someone who knows them of
old.

Production

After completing work on his Season Five adventure The Enemy Of The World, original Doctor
Who story editor David Whitaker was approached in 1968 by then-story
editor Derrick Sherwin about writing a new six-part serial concerning
humanity's first present-day encounter with alien life. The storyline --
apparently called “The Invaders From Mars” -- continued to be
developed into 1969, by which time Terrance Dicks had become Doctor
Who's script editor (as the post had been renamed) while Sherwin had
been promoted to producer.

On May 1st, Whitaker was commissioned to write a seven-episode storyline
under the title of “The Carriers Of Death”. By now, however,
it was known that the series' format would be undergoing a radical
redesign in the coming year; the Doctor would be regenerated and exiled to
Earth, and companions Jamie and Zoe would be replaced by Liz Shaw and
UNIT, the United Nations Intelligence Taskforce. In putting together his
storyline, then, Whitaker was asked to revise his original ideas for the
adventure in order to account for these changes. Full scripts were
requested on June 25th; “The Carriers Of Death” was at this
point scheduled to be the second story of Season Seven.

After David Whitaker turned in his first two scripts,
Derrick Sherwin decided that the approach was not what he wanted

Problems quickly developed with the serial, however. After Whitaker turned
in his first two scripts during July, Sherwin decided that the writer's
approach was not what he wanted. Assistant script editor Trevor Ray was
given the task of revising episode one, and this was sent to Whitaker on
August 11th to give him an idea of what the production team wanted. A
meeting was then held on the 13th to discuss the first two installments
and the in-progress part three. Work proceeded haltingly over the coming
months, however, with Whitaker and the production team unable to come to
terms with a suitable way of moving forward.

Finally, in mid-November, it was agreed that Whitaker would deliver
nothing past the third episode. Instead, Malcolm Hulke -- who had recently
completed The Silurians, the serial intended
to succeed “The Carriers Of Death” -- was commissioned on
November 18th to revise the delivered parts two and three, and write the
final four installments himself, making changes to Whitaker's storyline
where necessary. Whitaker, however, would maintain sole televised credit
on the serial. Furthermore, to allow time for Hulke to complete his task,
it was decided to transpose The Silurians
and “The Carriers Of Death” in the running order; the latter
therefore became Serial CCC.

Amongst the changes made to the adventure between Whitaker's original
storyline and Hulke's final drafts was the removal of an Army character
named Lieutenant Pollard from episodes two and three. As well, Professor
Heldorf was initially a German scientist named Kuhn, while Dobson was
called Dawson, Carrington was initially Cunningham, and Taltalian's name
was spelt “Taltalien”. As well, the two names on Reegan's van
were originally to have been Progressive Launderers Ltd and Masons Bakery,
but these were later changed to Hayhoe Launderers Ltd and Silcock Bakeries
in honour of assistant floor manager Margot Hayhoe and director's
assistant Pauline Silcock. Additional rewriting on “The Carriers Of
Death” was performed by Dicks and Michael Ferguson. Ferguson had
been assigned as the story's director, having most recently handled The Seeds Of Death some months earlier.

Although Letts was not entirely satisfied with Liz Shaw as a character,
Caroline John was contracted for the final two serials of Season Seven on
January 5th, 1970. Filming finally got under way on January 23rd. Two
Buckinghamshire locations were used: the Little Marlow Sewage Treatment
Works was the isotope factory, while the gravel pit was Folley's Gravel
Pit at Spade Oak. After the weekend, worked resumed with two days -- the
26th and 27th -- at the Southall Gas Works in Middlesex, which served as
Space Headquarters. Some warehouse footage was shot on January 27th on
nearby White Street, and the remainder was completed at TCC Condensers
in Ealing on the 27th and 28th.

Barry Letts was not entirely satisfied with Liz Shaw as a
character

Buckinghamshire was again the location of choice on the 29th. Wycombe Air
Park in High Wycombe served as Heldorf's lab, while the scenes of Liz
being pursued were filmed on roads in Marlow and at the Marlow Weir. John
was forced to wear a blonde wig in these sequences as her hair frizzed
badly in the rain. The retrieval of Recovery 7 was enacted at
Aldershot, Hampshire on January 30th and 31st. On February 2nd, nearby
Beacon Hill was the site of Reegan's lair. Finally, on the 3rd and 4th,
more Space Headquarters footage was captured at Blue Circle Cement Works
in Northfleet, Kent.

At about this time, the serial's title became The Ambassadors Of
Death. Studio taping then began on Friday, February 13th. Unlike The Silurians, on which new producer Barry
Letts had experimented with the recording pattern, The Ambassadors Of
Death employed the traditional studio schedule of completing one
episode every seven days. The first five installments were all taped in
BBC Television Centre Studio 3. Unusually, to save on costs, the
Recovery 7 set was a cofinanced venture between the Doctor
Who production office and the BBC drama series Doomwatch, for
which it served as Sunfire One in the episode Re-Entry
Forbidden.

For episode five, recorded on March 13th, the production team decided to
replace the role of the scripted sergeant (whose surname was apparently
West) with John Levene as Sergeant Benton. Benton had been introduced as a
Corporal in the previous season's The
Invasion, the story which had introduced the UNIT concept, and had
already been invited back for Inferno, the
next story in production. Recording shifted to TC4 for part six before
moving to TC1 for the last installment, taped on March 27th. Caroline
John's husband, Geoffrey Beevers, joined the serial for this episode,
playing Private Johnson.

The Ambassadors Of Death was Whitaker's last, and unfortunately
least favourite, Doctor Who story. It was also amongst his final
BBC work, after which he continued writing, including a stint in
Australia. Whitaker took ill shortly after agreeing to novelise The Enemy Of The World for Target Books. Sadly,
it was to be a project Whitaker would never see through to completion, as
he died of cancer on February 4th, 1980.

Sources

Doctor Who: The Handbook: The Third Doctor by David J Howe and
Stephen James Walker (1996), Virgin Publishing, ISBN 0 426 20486 7.